Flashcards in Disorders of calcium, phosphate and magnesium. Deck (49):
What is the physiological importance of calcium?
- Muscle contraction
- Neuronal excitation
- Enzyme activity (Na/K ATPase, hexokinase etc.)
- Blood clotting
What is the structural importance of calcium?
Key component of hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ - the predominant mineral in bone
What is the physiological importance of phosphate (PO4-)
- The P in ATP – our fuel!
- Intracellular signalling
- Cellular metabolic processes e.g. glycolysis
What is the structural importance of phosphate?
- Backbone of DNA
- Component of hydroxyapatite Ca₁₀(PO₄)₆(OH)₂
- Membrane phospholipids
What is the physiological importance of magnesium?
- Cofactor for ATP
- Neuromuscular excitability
- Enzymatic function
- Regulates ion channels
What is the normal range for calcium?
2.20 -2.60 mmol/L
What are the two key controlling factors for calcium?
- Vitamin D and metabolites
What processes is calcium related to/involved in?
- GI uptake
- Renal Clearance
What is plasma calcium?
Total Ca = Ionised Ca + Bound Ca + Complexed Ca
What is ionised calcium?
Physiologically active fraction
- Calcium sensing receptor
- Cellular Effects
- Regulation of PTH
What is bound calcium?
- Physiologically inactive
- Albumin main binding protein (~50%)
What is complexed calcium?
Salts – calcium phosphate & calcium citrate
What can calcium values be corrected for?
Changes in albumin. Better diagnostic performance.
What effect does acidosis have on Ca-albumin?
What effect does alkalosis have on Ca-albumin?
What can alkalosis precipitate?
What are the endogenous factors related to Vit D absorption?
- Skin colour
What are the exogenous factors related to Vit D absorption?
- Sunscreens & clothing
- Latitude and season
- Time of day
- Diet & supplements
What factors cause vitamin D to fail?
- Body fat
What is the role of parathyroid hormone (PTH)?
Acts on bone, GI and kidney to increase levels of ionic calcium. Increased ionic Ca acts upon the parathyroid glands to stop the release of PTH.
What is the effect of PTH on the kidney?
Decreases Ca clearance but increases phosphate excretion.
What is the effect of PTH on bone?
Increases Ca turnover with net resorption.
What is the effect of PTH on the gut?
Stimulates the kidney to release vit D.
Vit D increases Ca absorption from the gut.
What are the causes of hypocalcaemia?
- Vit D deficiency
- Inadequate intake of Ca
- Artefactual causes
What are the causes of Vit D deficiency?
- Dietary / Malabsorption
- Hepatic disease
- Renal Disease
- End organ Vitamin D resistance
What are the causes of hypoparathyroidism?
- Secondary to Mg depletion
What is the cause of pseudohyoparathyroidism?
End organ PTH resistance.
What are the artefactual causes of hypocalcaemia?
- EDTA contamination
- Venestasis will cause low adjusted calcium (total calcium is unaffected)
How can you tell the difference between hypoparathyroidism and Vit D deficiency?
Vit D deficiency - High PTH, low Ca
Hypoparathyroidism - rock bottom PTH, low Ca
What are the possible causes of hypercalcaemia?
- Lytic lesions
- Humoral eg PTHrp
Vitamin D excess
- 1a cholecalciferol
Bone disease + immobilisation
What is the most common cause of ludicrously serum high potassium.
What are the differential diagnoses of hypercalcaemia?
- Consider Adjusted Ca: look at the albumin
- Check drug history
- Exclude excess vitamin D intake
- Check for renal failure
- Simultaneous measurement of Ca & PTH
- Consider rarer causes & more complex investigations
How can you tell the difference between hyperparathyroidism and bone metastases?
Hyperparathyroidism - high PTH, high calcium
Metastases - low PTH, high calcium
What are the causes of phosphate deficiency?
- renal tubular damage
- Gastro intestinal
- diabetes (diuresis)
- inadequate IVN
ECF / ICF redistribution - like potassium
What are the signs/symptoms of phosphate deficiency?
- Haemolysis, thrombocytopenia and poor granulocyte function
- Severe muscle weakness, respiratory muscle failure and rhabdomyolysis
- Confusion, irritability and coma may be due to a metabolic encephalopathy due to phosphate deficiency
- Renal dysfunction
What is hypomagnesaemia associated with?
Hypokalaemia Hyponatraemia Hypophosataemia
What are the renal causes of magnesium depletion?
- Diuretic phase of Acute
- Tubular Necrosis
- Hypercalcaemic states
-- Antibiotics - gentamicin/carbenicillin
-- Chemotherapy - cisplatin
What are the GI causes of magnesium depletion?
- Intravenous nutrition
What are the cellular effects of magnesium depletion?
- Reduced mitochondrial respiration & impaired phosphorylation
- Defective Na-K ATPase activity
- Impaired DNA synthesis
What are the biochemical effects of magnesium depletion?
What are the endocrine effects of magnesium depletion?
- Impaired PTH release
- PTH resistance in bone
What are the cardiovascular effects of magnesium depletion?
- Cardiac irritability
- Reduced contractility
What are the CNS effects of magnesium depletion?
- Ataxia / Vertigo
- Psychosis / Depression
What are the muscle effects of magnesium depletion.
- Muscle fibrillation
- Myopathic EMG
What is the relevance of serum magnesium?
- Poor correlation with Mg status
- Of value acutely
What is the relevance of erythrocyte Mg?
- Poor correlation with Mg status
- Affected by high reticulocyte count
What is the relevance of leukocyte Mg?
- Good correlation
- Good predictive value
What is the relevance of muscle Mg?
- 20% body mass
- important physiologically
- requires biopsy